Small sized electric appliances for personal care often include functional elements or working tools driven by an electric-type, more particularly magnetic-type drive unit which may be received within a housing element that may form a handpiece to be handheld.
For example, electric shavers may have one or more cutter elements driven by an electric drive unit in an oscillating manner where the cutter elements reciprocate under a shear foil, wherein such cutter elements or undercutters may have an elongated shape and may reciprocate along their longitudinal axis. Other types of electric shavers use rotatory cutter elements which may be driven in an oscillating or a continuous manner. The said electric drive may include an electric motor or a magnetic type linear motor, wherein the drive unit may include an elongated drive transmitter for transmitting the driving motion of the motor to the cutter element.
Such drive systems include sometimes linear-type drive units comprising first and second drive components reciprocating or oscillating relative to each other in a substantially linear manner, i.e. substantially along linear axes, wherein the driving forces may result from magnetic fields. For example, one of the drive components may include a permanent magnet, whereas another one of the drive components may include one or more magnetic coils to which pulsating electric current is applied to create pulsating magnetic fields, thereby causing the two drive components to oscillate relative to each other. At least one of the drive components connects to a transmission train transmitting the oscillating movement of the drive component onto the functional element to be driven such as the aforementioned cutter element, wherein such transmission train may include a transmitter pin directly connecting to the cutter element or indirectly connected thereto by means of a yielding bridge structure allowing for pivoting movements of the cutter element.
For example, US 2009/0025229 A1 or U.S. Pat. No. 7,841,090 B2 discloses an electric shaver having a pair of cutter elements provided under a shear foil and driven in an oscillating manner.
Furthermore, WO 03/103905 A1 and EP 0 674 979 A1 disclose linear oscillating drive units for shavers, wherein the drive components oscillating relative to each other in a linear manner include a permanent magnet on the one hand and a magnetic coil on the other hand.
In such systems, one of the drive components may be rigidly connected to a mounting structure or the installation environment which is often a handpiece formed by a housing part of the electric appliance in which the drive unit is received. For example, the permanent magnet may be rigidly supported or fixedly connected to an interior side of the handpiece via a drive carrier or a mounting structure connected thereto, whereas the other drive component including the magnetic coils may be movably supported on said drive carrier for allowing the linear oscillation, for example by means of a pendulum bearing. Due to the fixed connection of one of the drive components to the handpiece, undesired vibrations can be implied onto the handpiece, thereby reducing the handling comfort.
So as to reduce such undesired vibrations onto the handpiece, it already has been suggested to support both drive components displaceable relative to said mounting structure to allow both drive components to execute linear oscillation in a counteracting manner. For example, WO 03/103905 A1 suggests to not fix one of the drive components, but to fix the linkage or pendulum bars linking the two drive components to each other, to the drive carrier and thus, to the installation environment in terms of an inner portion of the handpiece housing. Such fixing of the pendulum bearing to the drive carrier allows both drive units to oscillate in the direction of the oscillation axis in a sort of reverse motion. When a first drive component moves to the left, the other drive component moves to the right, and vice versa. Such reverse oscillation may reduce the aforementioned undesired vibrations of the handpiece.
However, due to tolerances of the drive components and/or phase offset, there still may be mismatch of the dynamic effects of the reverse motions and thus, vibrations that can be felt in the hand holding the handpiece. Such phase offset between the oscillating components of the drive unit may be caused, for example, by frictional effects between the shear foil and the cutter elements. When the components are not oscillating in exact 180° phase opposition, vibrations arise. Moreover, when there are unbalanced or different loads onto the oscillating motor components, for example when there is only one element or component under load, the desired dynamic compensation may no longer be achieved.
Furthermore, due to restrictions of the oscillating amplitude of the drive components and restrictions in the mounting space, the cutter elements at the shaver head sometimes cannot be driven at the desired oscillation amplitude. As the same type of drive unit is sometimes used for different types of shavers to keep manufacturing costs low by means of applying modularity, the oscillation amplitude provided by such common drive unit does not fit all shaver types so the cutter element of a specific shaver type is not driven at the desired oscillation amplitude. Moreover, it is sometimes desired to have higher driving forces at smaller oscillation amplitudes or in the alternative lower driving forces at higher amplitudes, and thus to have a decreasing or increasing transmission ratio.